Securities (Narrative) (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) Investment	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) Investment	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment security gains, net	$ 0.4	$ 2.4	$ 0.7	$ 2.1
Gross proceeds from sale of securities	978.5	10.0	1,500.0	523.6
Changes in other-than-temporary-impairment (OTTI) losses	$ 0.0	$ (2.4)	(0.1)	(2.4)
Gross realized securities losses			0.8
Gross realized securities gains			$ 0.1	$ 0.3
Number of securities in an unrealized loss position | Investment	1,077		1,077
Total fair value	$ 21,056.6		$ 21,056.6		$ 22,040.2
Total unrealized losses	153.6		153.6		$ 176.0
Government Sponsored Agency
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	61.6		61.6
Corporate Debt
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	$ 19.9		$ 19.9
Percent of corporate debt portfolio	37.00%		37.00%
Other | Community Reinvestment Act CRA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	$ 36.3		$ 36.3
Auction Rate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	$ 0.3		$ 0.3